In a number of modern two-way radio and cellular telephone products, zero intermediate frequency (ZIF) technology is used in the radio receiver. The ZIF operates using the incoming radio frequency signal where it is mixed or heterodyned with an internal local oscillator (LO) at approximately the same frequency. A resultant or intermediate frequency signal is created by this process. This intermediate frequency is at approximately 0 hertz. The intermediate frequency signal is then passed through a respective low pass filter, demodulator, and audio processing circuits where it is output in the form of a recovered audio signal available to a user. This process is generally referred to as direct conversion.
Since the LO and desired frequency are roughly equal, the LO can also conveniently be used to transmit a signal at that same frequency. Thus, a common LO synthesizer can be used to both transmit and receive in the direct conversion two-way radio transceiver. This technique needs only a single voltage controlled oscillator (VCO). In the radio transmitter portion of the transceiver, the VCO is typically run at twice the desired signal frequency and later divided by two down to the desired frequency. When used in this manner, this simple system is ideal for integration onto a single microelectronic silicon chip-commonly known as an integrated circuit (IC). Of course before the transmit signal is passed to a RF power amplifier, the signal is generally buffered and amplified.
Unfortunately problems occur with such a localized system where many RF signals are used in such a manner. The close circuit proximity facilitates signal cross coupling through the common silicon substrate. Therefore one system can adversely react with the other. Additionally, problems also can occur in the amount of RF feedback through circuit parasitics and this can also disrupt the operation of the VCO or synthesizer. The transmit buffer output signal, typically on the order of +5 dBm, must be isolated from the VCO output signal having an amplitude of -20 dBm or less. Thus, a significant amount of RF isolation is necessary to keep the large amplitude of the transmit buffer from interfering with the small VCO signal. The integrated circuit's signal nodes will generally have some parasitic capacitance to a common substrate or node. It is by this common node that the various oscillator and synthesizer circuits interact with one another. By decreasing the resistance at a given circuit's node, the substrate and circuit parasitic coupling can be reduced. This is achieved by using the tx buffer in a current mode operation.
Moreover, each time the IC package pin count is increased, or any external parts are needed in the design of a transmit buffer, an incremental construction cost will occur. This is due to the increase in manufacturing complexity and total part count. Additionally, in many transmit buffer designs external bypass parts are necessary because they are difficult to integrate onto silicon. Further, the bandwidth of the oscillator will depend upon the selection of these devices. Different external parts are often necessary to supply a variation in frequency ranges. This tends to even further complicate already complex manufacturing techniques. External connections other than the output of the IC are highly undesirable in an IC transmit buffer circuit.
Therefore, the need exists to provide a transmitter circuit where the transmit signals on a single chip IC design remain locally isolated while allowing wideband operation, with inexpensive full system integration.